I have 9023187 which I took delivery on 02-Sep-09 from Amazon.com. I haven't done the brick wall test yet, but my wife & daughter weren't impressed with those first photos! (brushed concrete, 4 focal ranges, 5 aperatures, focusing distance of approx. 2 feet)

They just couldn't believe I took 20 photos of concrete.
I have the slight resistance on the zoom ring at about 28mm - no biggie.
I ran a AF test and the results appear quite good - I will run the same test on bigger brother (DA*50-135) before I make a final decision on that. I noticed a bit of a "squeak" when the lens found the focus distance at first; but after a couple hours of use, this has improved.
I took some real world photos today and compared to a SMC PENTAX-M ZOOM 1:3.5 28mm~1:4.5 50mm. It was a good photo day, partly cloudywith lots of contrast, but the light conditions were changing fast (mostly sunny with fast moving cumulous clouds). This made comparison difficult in terms of color saturation & vibrancy, but in regards to sharpness, the DA* was a bit better than the M ZOOM. I plan to run some head to head tests with a SMC PENTAX-M 1:2.8 28mm & the SMC PENTAX-M 1:1.4 50mm soon.
I have seen some evidence of decentralization with the brushed concrete photos, but it is in the corners only. In landscape mode, the lower right corner is a softer than the other three corners. It is nothing even close to the 10 - 25% of the image being soft on one side or the other. I can also see some indication of decentralization when using a polarizing filter at the 16mm focal length - the vignetting is noticeable (and even) on both left hand corners, somewhat less in the lower right, and only slight in the upper right.
In regards to vignetting, I see more at 50mm than 16mm, but have run no quantitative tests as yet.
I think this thread scared me!

27 pages of horror stories with a few "I got a good one" thrown in there without much further comment. So far, I cautiously optimistic.
When I bought the K20D earlier this year paired with the DA*50-135, the photos were so good, sharp, & vibrant that I didn't even consider testing the lens in a thorough, quantitative way. When I started considering the purchase of the DA*16-50, I was hoping for that same quality. But, I felt that the quality history of the DA*16-50 really made it gamble to shell out the $700.